How Can I Combine Separate Date and Time Columns into a Single DateTime Column in MS SQL Server?

Merging Date and Time Columns in MS SQL Server: A Comprehensive Guide

Many database designs store date and time information across two separate columns. This guide explains how to efficiently combine these into a single datetime column within MS SQL Server.

The Solution

Assuming your "Date" column contains only date values (time portion is zero) and your "Time" column contains only time values (date portion is January 1, 1900), a simple addition operation surprisingly works:

<code class="language-sql">SELECT CombinedDateTime = MyDate + MyTime FROM MyTable;</code>

Understanding the Mechanism

This seemingly straightforward addition leverages the internal representation of datetime values in older SQL Server versions. Each datetime value is stored as two 4-byte integers: one for the date and one for the time. Adding them performs a bitwise combination, resulting in a single datetime value.

Handling Newer SQL Server Versions and Data Types

For SQL Server 2008 and later, the DATE and TIME data types have been introduced, alongside DATETIME and DATETIME2. Direct addition no longer applies. To combine DATE and TIME in these newer versions, explicit casting to DATETIME is crucial:

<code class="language-sql">SELECT CombinedDateTime = CAST(MyDate AS DATETIME) + CAST(MyTime AS DATETIME) FROM MyTable;</code>

Furthermore, directly adding datetime2 values can lead to precision loss. To ensure accuracy, convert to DATETIME before combining:

<code class="language-sql">SELECT CombinedDateTime = CAST(MyDate AS DATETIME) + CAST(MyTime AS DATETIME) FROM MyTable;</code>

This approach guarantees a precise and reliable combination of your date and time components into a single datetime column, regardless of your SQL Server version.

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